Measuring Body Temperature with an Infrared Sensor – IOThrifty

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Measuring Body Temperature with an Infrared Sensor

Using Infrared Temperature Sensor for Body Temperture Measurement

Since the outbreak of the Covid-19 Corona virus pandemic one of the more common application questions we receive is “Can an infrared temperature sensor be used for body temperature measurement?”  Our sensors are  designed for industrial and laboratory surface temperature measurement not fever detection or medical use.   If you are investigating using an infrared sensor for body temperature, this blog discusses some of the the issues to be considered.

 

What is the Required Complete System Accuracy?

A typical accuracy for an infrared temperature sensor is ±1°C. If that level of accuracy is sufficient then the sensor may be a good choice for the application.  It may also be possible to get a little greater precision than the absolute accuracy by conducting a calibration at the specific target temperature. Many sensors have a better repeatability than accuracy and by calibrating at the target temperature you may be able to yield an accuracy closer to the repeatability at the target temperature. This is more challenging over a wide temperature range but is a possibility for a narrow temperature range like body temperature.  Also, the complete system accuracy needs to be considered, not just the sensor accuracy. For example, a sensor with a ±1°C accuracy and digitized output such as USB or RS485 will have a system accuracy equal to the accuracy of the sensor since there is not further processing required to produce a reading in temperature units.   However, if the output of the sensor is an analog signal such as 4-20mA or 0-10Vdc there is additional processing required to convert the sensor output to temperature units. The processing of the analog signal will add some error into the final reading.   An extreme example of this is our infrared thermocouples.  These sensors self-powered, compact, and very accurate (better than 0.2°C at the target temperature) but they produce a thermocouple output.  There is lot of commercial instrumentation that will convert the thermocouple signal into a temperature reading but most off the shelf equipment will add 1°C or more of error to the measurement  A custom instrument design can produced a more precise result but the cost of the design has to be factored into the overall system cost.

 

Optical Field of View and Targeting the Area to Measure

All infrared temperature sensors have and optical field of view which defines the size of the target it measures. A simple analogy is the circular area produced when you shine a flashlight on a wall.  The closer you move the flashlight to the wall the smaller the lighted area becomes.  The further you move the flashlight from the wall the larger the area becomes.  An infrared sensor does not produce a beam of light, but it measures the temperature the target emits via infrared energy in a conical shape.   The target area which the sensor “sees” is determined by the optics of the sensor and and the distance of the target from the sensor.   The drawing below is for one of our infrared sensors with 20:1 field of view optics.  This sensor will measure the temperature of a 1.5-inch spot at 19.7 inches from the sensor. The closer the target is to the sensor the smaller the area and the further the sensor from the target the larger the area.   The size to distance ratio is a specific characteristic of this sensor, known as the field of view.  Other sensors may have similar or quite different field of views.

 

If you were to move the sensor further from the target the spot size would increase.  Although there is no practical limit on how far the sensor can be from the target, if the spot size is larger than the target, the sensor will incorporate the temperature of anything in the back ground into the reading resulting in a bad reading.  This brings up the challenge of targeting.  If the sensor is used to measure the temperature of a person’s forehead, the measurement spot must be fully contained on the forehead.  Not a simple task if the person is moving or the exact distance from the sensor is not controlled.  In industrial applications, a sensor with built in laser (similar to a laser pointer) may be used for targeting.  But this would not be recommended in a body measurement application because of the damage the laser may do if pointed into someone’s eyes.

 

Sensor Output

We have already talked about the sensor output in the context of accuracy, but it is worth mentioning here again as a stand-alone consideration.  Typical outputs and the advantages and disadvantages are shown in the table below.

Output

Advantage/Disadvantage

Thermocouple

Integrates well with existing temperature measurement systems.  Even though sensors may have a high degree of accuracy the conversion to temperature measurement often introduces ±1°C or more of error

Current (eg. 4-20mA)

Good for long transmission distances.  Some error introduced by converting to temperature but that is typically minor.

Voltage (eg.0- 5Vdc, 0-10Vdc)

Good for medium transmission distance.  Easy to convert to temperature with minimal introduced errors.

USB

Normally limited to short transmission distances.  Only one sensor on each USB connection.  No error introduced by conversion to temperature units.

RS485

Good for long transmission distances, multiple sensors can be on one connection. No error introduced by conversion to temperature units

 

Is a Surface (Skin) Body Temperature Measurement a Good Indication of Body Temperature?

Infrared temperature sensors measure the bodies outer skin temperature  but whether that is a good indication of internal body temperature is a medical question and beyond the scope of this blog.  One factor which may influence the reading is ambient environment temperature. That is, a cold or hot ambient environment could lower or raise the measured temperature, respectively.   Even a superficial Google search will find many articles discussing how skin temperature will often be different temperature than internal  to body temperature (here is one example https://hypertextbook.com/facts/2001/AbantyFarzana.shtml) .  Because of this and many other considerations, IOThrifty makes no guarantees or recommendations that infrared temperature sensors should be used as a valid indication of a person’s true internal body temperature.  This blog was written to help the reader understand the many considerations that need to be addressed if using an infrared sensor in this application.


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